Various types of frequency converter systems have been known to the prior art. One specific application of such a frequency converter system is the time compression or expansion of an audio, video or computer signal normally stored in some sort of archival system, such as tape, magnetic or optical disk or memory.
If time compression is desired for a stored signal, the stored signal is produced at an increased speed to reduce the total duration of the playback time of the stored signal. Unfortunately, when the stored signal is produced at an increased speed, the signal undergoes an increase in frequency relative to the stored signal being produced at a normal speed.
Conversely, when the stored signal is produced at a decreased speed to expand the signal playback time, the signal undergoes a decrease in frequency relative to the stored signal being produced at a normal speed.
Under either circumstance it would be desirable to provide the ability to change the frequency of the produced signal by a percentage function based on the ratio between the lengths of time of production. Alternately one can change the frequency of a real time signal (i.e., without altering the time period of production).
For example, it might be desirable to replay a prerecorded thirty (30) minute television program in a time duration of twenty-eight (28) minutes in order to fit an allocated time slot without the associated seven percent (7%) increase in frequency. The replay of a prerecorded thirty (30) minute television program in twenty-eight (28) minutes would allow alternately for the insertion of an extra two (2) minutes of commercials. Unfortunately, a viewer can discern the seven percent (7%) increase in pitch resulting from an increased speed in the replay of the signal. Alternatively, a taped show may run only 45 minutes, with the station having a 50 minute time slot. The present invention allows the show to be expanded to fit the time slot.
In another example, entertainment or educational programs or movies could be presented in shorter time to reduce the operating costs of a movie theater or to allow more movies to be shown in an evening. A similar advantage could be realized in the replay of prerecorded music or voice on a radio station. Messages from an answering machine could be accelerated, perhaps greatly, for rapid playback while retaining normal voice frequencies. Again, the present invention removes the pitch shift artifacts that would otherwise be ascertainable to the consumer.
A further example, some live talk shows have a six (6) second profanity dump memory (that allows the selective deletion of expletives). Some of these dumps, however, also can produce an audio gap after the expletive is deleted due to the need to fill up their memory line with new audio information. The present invention allows for an effective instantaneous switch back to live audio, since delay can be gradually re-accumulated without pitch change.
Another example would be to change the relative pitch of the human voice so as to allow an individual to sing harmony with themselves in real time.
Another example would be to lower the occupied bandwidth of a signal to be transmitted over a radio propagation or other transmission medium.
A last example wherein data may be fed in to a memory, perhaps intermittently, at one speed and fed out at a second (normally slower and perhaps constant) speed, thus facilitating computer operation or elementary data operations.
Some in the prior art have attempted to develop a frequency converter system with varying degrees of success. Although some of these frequency converter systems functioned properly, many of these frequency converter systems were excessively complex and costly to manufacture. Accordingly, the frequency converter systems of the past have not found wide use in the media art.
Frequency converter systems of the prior art include U.S. Pat. No. 4,829,257 to J. Carl Cooper for an improved device for accurately phase or frequency shifting an input signal. This invention incorporated a variable resistor extending between at least two known phase shifted values of the input signal. U.S. Pat. No. 4,868,428 to J. Carl Cooper discloses an apparatus and method for accurately shifting the phase or frequency of a complex signal. U.S. Pat. No. 5,097,218 to J. Carl Cooper discloses an apparatus and method for accurately multiplying the phase or frequency of complex time varying signals by a given factor which may be non-integer.